The present invention relates to powder of iron-base alloy permanent magnets that are suitable as magnets used for electrical appliances such as motors, actuators, speakers, meters, and focus/convergence rings, and a method for producing such powder. The present invention also relates to bonded magnets manufactured from the above powder, and various types of electrical appliances provided with the bonded magnets.
An Fexe2x80x94Rxe2x80x94B alloy nanocomposite magnet is an iron-base alloy permanent magnet where crystallites of a soft magnetic Fe boride such as Fe3B and Fe23B6 and crystallites of a hard magnetic R2Fe14B phase are uniformly distributed in the same metal structure and magnetically coupled to each other as a result of exchange interactions therebetween.
Although the nanocomposite magnet contains the soft magnetic crystallites, it exhibits excellent magnetic properties by the magnetic coupling of the soft magnetic crystallites with the hard magnetic crystallites. In addition, since the soft magnetic crystallites do not include any rare-earth element (R) such as neodymium, the total amount of rare-earth elements contained in the magnet is small. This reduces the production cost and thus is suitable for stable supply of the magnet.
The nanocomposite magnet of this type is produced by solidifying a molten material alloy by melt quenching and then appropriately heat-treating. A single roll method is often employed for quenching a molten material alloy, where a molten material alloy is brought into contact with a rotating chill roll to be chilled and solidified. According to this method, the quenched alloy is in the shape of a strip (or ribbon) elongated in the direction of the circumferentially rotating direction of the chill roll.
Conventionally, the roll is operated at a surface velocity of 15 m/sec or more to produce a quenched alloy strip having a thickness of 50 xcexcm or less. The resulting quenched alloy strip is heat-treated to impart permanent magnet properties, and then pulverized to obtain magnet powder having an average particle size of 300 xcexcm or less. The magnet powder is then formed into a permanent magnet having a desired shape by compaction or injection molding.
When a relatively thin quenched alloy strip having a thickness of 50 xcexcm or less is pulverized to obtain powder having an average particle size of 300 xcexcm or less, the resultant powder particles are flat in shape. Such magnet powder produced by the above conventional technique is therefore poor in packing density and flowability, with the magnet powder packing density being 80% at maximum for compaction and 65% for injection molding. The magnet powder packing density influences the properties of the permanent magnet as a final product. It is therefore strongly desired to increase the magnet powder packing density for improvement of the properties of the permanent magnet.
A primary object of the present invention is providing iron-base alloy permanent magnet powder with improved packing density and flowability during compaction, of which particles have a roughly spherical shape, not a conventional flat shape, and a method for producing such iron-base alloy permanent magnet powder.
It is also an object of the present invention to provide a bonded magnet having excellent permanent magnet properties manufactured using the above iron-base alloy permanent magnet powder with improved packing density, and electrical appliances including such a bonded magnet.
The method for producing iron-base alloy permanent magnet powder of the present invention includes the steps of: chilling an Fexe2x80x94Rxe2x80x94B molten alloy by melt quenching, thereby forming a rapidly solidified alloy having a thickness in a range of 80 xcexcm to 300 xcexcm; crystallizing the rapidly solidified alloy by heat treatment, thereby producing an alloy having permanent magnet properties; and pulverizing the alloy to produce powder having an average particle size in a range of 50 xcexcm to 300 xcexcm and a ratio of minor axis size to major axis size of powder particles in a range of 0.3 to 1.0.
In a preferred embodiment, the method further includes the step of coarsely pulverizing the rapidly solidified alloy before the heat treatment. The step of pulverizing is preferably carried out with a pin disk mill.
In another preferred embodiment, the rapidly solidified alloy includes at least one metastable phase selected from the group consisting of Fe23B6, Fe3B, R2Fe14B, and R2Fe23B3 and/or an amorphous phase, before the heat treatment.
In yet another preferred embodiment, the alloy having permanent magnet properties is a permanent magnet represented by a general formula, Fe100-x-yRxBy (R is at least one rare earth element selected from the group consisting of Pr, Nd, Dy, and Tb), wherein x and y in the general formula satisfy the relationships of 1 at %xe2x89xa6xxe2x89xa66 at %, and 15 at %xe2x89xa6yxe2x89xa625 at %, and the alloy contains iron, an alloy of iron and boron, and a compound having a R2Fe14B crystal structure as component phases, the average crystal grain sizes of the component phases being 150 nm or less.
In the chilling step, the molten alloy is preferably brought into contact with a roll rotating at a surface velocity in a range of 1 m/sec to 13 m/sec, thereby forming the rapidly solidified alloy.
The chilling step preferably includes the step of quenching the Fexe2x80x94Rxe2x80x94B molten alloy in a reduced atmosphere. The absolute pressure of the decompressed atmosphere is preferably 50 kPa or less. Preferably, the alloy obtained by the crystallization by heat treatment is a nanocomposite magnet.
The method for manufacturing a bonded magnet of the present invention includes the steps of: preparing iron-base alloy permanent magnet powder produced by any of the methods for producing iron-base alloy permanent magnet powder described above; and compacting the ion-base alloy permanent magnet powder.
In a preferred embodiment, the iron-base alloy permanent magnet powder is compacted at a packing density exceeding 80%.
In another preferred embodiment, the iron-base alloy permanent magnet powder is molded by injection molding at a packing density exceeding 65%.
The iron-base alloy permanent magnet powder of the present invention is represented by a general formula, Fe100-x-yRxBy (R is at least one element selected from the group consisting of Pr, Nd, Dy, and Th), wherein x and y in the general formula satisfy the relationships of 1 at %xe2x89xa6xxe2x89xa66 at %, and 15 at %xe2x89xa6yxe2x89xa625 at %. The powder contains iron, an alloy of iron and boron, and a compound having a R2Fe14B crystal structure as component phases. The average crystal grain sizes of the component phases is 150 nm or less. The average particle size of the powder is 300 xcexcm or less, and a ratio of minor axis size to major axis size of powder particles is in a range of 0.3 to 1.0.
The bonded magnet of the present invention includes the iron-base alloy permanent magnet powder described above. The packing density exceeds 80% of maximum when the bonded magnet is manufactured by compaction, and it exceeds 65% of maximum when the bonded magnet is manufactured by injection molding.
The electrical appliance of the present invention is provided with the bonded magnet described above.